Transducer drive circuit and signal generator

ABSTRACT

A transducer such as a piezoelectric element forms part of the diaphragm of a speaker in a sonic signal generator. A timing circuit and the piezoelectric transducer element are coupled to a level detector and voltage comparing circuit. This circuit monitors the phase and voltage of the piezoelectric transducer and compares the same with the signal from the timing circuit. When a selected condition is reached a controlled switch connected between the transducer and the power supply is operated to cause a small signal to be applied to the transducer at the appropriate time to maintain the transducer in an oscillatory state corresponding to its resonant frequency with the expenditure of a very small amount of energy from the power supply.

United States Patent [191 Proctor 1 June 26, 1973 TRANSDUCER DRIVECIRCUIT AND SIGNAL GENERATOR [76] Inventor: Darryl Frederic Proctor,17402 NE.

22nd, Bellevue, Wash. 98052 [22] Filed: Jan. 11, 1971 [21] Appl. No.:105,583

331/116 R, 116 M, 154, 155, l72-l74;310/8, 8.1; 317/262; 332/263,621,469 11/1971 Bauer ..331/1l1 Primary Examiner-John W. CaldwellAssistant Examiner-William M. Wannisky Attorney-Christensen & Sanborn[57] ABSTRACT A transducer such as a piezoelectric element forms part ofthe diaphragm of a speaker in a sonic signal generator. A timing circuitand the piezoelectric transducer element are coupled to a level detectorand voltage comparing circuit. This circuit monitors the phase andvoltage of the piezoelectric transducer and compares the same with thesignal from the timing circuit. When a selected condition is reached acontrolled switch conl l Relerences Cited nected between the transducerand the power supply is UNITED STATES PATENTS operated to cause a smallsignal to be applied to the 3,569,963 3 1971 Mallory 340/384 E xtransducer at the pp p time to maintain the 3,324,403 6 1967 Chapman e1, 340 334 5 x transducer in an oscillatory state corresponding to its3,239,776 3/1966 Shaw 331/116 R resonant frequency with the expenditureof a very small 3,596,206 7/1971 Loria 331/116 R ount of energy from thepower supply. 3,559,158 l/l97l Bettcher 340/384 E 3,398,380 8/1968 Dwyer331/116 R 13 Claims, 7 Drawing Figures 3 v /0 PIE ZOEL ECTR/C TRANSDUCERT/MI/YG POWER 0/?50/7 SUPPLY [El/EL CONTROLLED DETECTOR \S'W/TCHTRANSDUCER DRIVE CIRCUIT AND SIGNAL GENERATOR BACKGROUND OF THEINVENTION Audio signal generators are widely used at the present time,as for example in thetelephone art. In such applications it is ofimportance to be able to operate a substantial number of the telephoneringing circuits without imposing an undue drain on the power supply.Various attempts have been made to provide these and other electricallyoperated audio generators using as little energy as possible whileobtaining substantial audio energy output. Thus sonic transducers suchas disclosed in US. Pat. No. 3,331,970 have been devised wherein atransducer such as a piezoelectric crystal is connected to a thinmetallic diaphragm forming part of an audio speaker. In such anarrangement audio signals corresponding to the resonant frequency of thepiezoelectric element are generated. Various other uses of transducerelements such as piezoelectric devices are well known, particularlythose wherein the transducer is driven at its resonant frequency.

An oscillator carefully designed to have exactly the same frequency asthe piezoelectric transducer has been typically used as the drivingcircuit in applications such as mentioned above. U. S. Pat. No.3,277,465 to Potter suggests such an approach. While a carefully tunedoscillator can be used to drive a transducer, I

have found that the energy required to drive the transducer and theoscillator itself is excessive, and in fact will severely limit thenumber of ringing devices which can be driven by a single source.

It is therefore an object of the present invention to provide anefficient transducer drive circuit. Another object of the invention isto provide a driving circuit for a piezoelectric transducer whichcircuit requires an extremely small amount of electrical operatingenergy.

Another object of the present invention is to provide an audio signalgenerator utilizing a piezoelectric element in combination with acontrolled power switch which provides a small boost to thepiezoelectric element at a selected point in the physical movement ofthe piezoelectric element.

SUMMARY OF THE INVENTION In accordance with the teachings of the presentinvention a piezoelectric transducer is connected to a power supply sothat when power is applied to the element the element will be distortedin the manner well known in the art and attempt to vibrate at itsresonant frequency. At the time of applying energy to the piezoelectricelement a timing circuit is also provided with power. The timing circuitand the piezoelectric element are connected to a level detecting andcomparing circuit which compares the voltage level of the piezoelectricelement with the level of the signal from the timing circuit. The timingcircuit is adjusted such that its voltage level achieves a selectedamplitude at a point in time corresponding to the signal of thepiezoelectric element approaching a maximum. At this point in time acontrolled switch circuit connected to the piezoelectric element and tothe power supply is actuated so that a very small signal is applied tothe piezoelectric element. The applied signal is of the proper polarityand accurately timed so that only a small boost signal is applied to thepiezoelectric element, causing it to swing through its maximum signalcondition. The controlled switch then turns off and the transducercontinues on through another cycle, the timing circuit being reset foranother cycle of operation.

By thus monitoring the voltage of the piezoelectric element andaccurately timing the application of a small boosting signal,thepiezoelectric element is maintained in a vibrational modecorresponding to its inherent resonant frequency. The system is suchthat the amount of energy applied to the piezoelectric element ismaintained at a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS The above and additional advantagesand objects of the invention will be more clearly understood from thefollowing description when read with reference to the accompanyingdrawings.

FIG. 1 is a block diagram illustrating the principles of a preferredembodiment of the present invention.

FIGS. 2, 3, 4, 5 and 6 are schematic circuit diagrams illustratingpreferred embodiments of the present invention.

FIG. 2A is a waveform diagram illustrating voltages appearing atselected points in the circuit of FIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawingsthere is illustrated in FIG. 1 a piezoelectric transducer 10 which isprovided with energy from the power supply 11 whenever the main switch12 is closed. The power supply 11 also applies power to the timingcircuit 13 and to the controlled switch 14 when the main switch 12 isclosed. A level detector and comparison circuit 15 is connected to thetiming circuit 13 and to the transducer 10 and serves to compare thevoltage level appearing on the transducer 10 with the voltage of asignal from the timing circuit 13. When the voltage levels aresubstantially equal, the controlled switch 14 connected to the detectorand comparator 15 is closed. The controlled switch 14 is connected bycircuit 16 to the piezoelectric transducer 10 with the arrangement beingsuch that closure of the switch 14 causes a signal to be applied to thetransducer 10 at the appropriate time in the cycle of movement thereofto assure continued vibration of the transducer 10. The transducer 10 ispreferably a part of a sonic signal generator such as disclosed in U. S.Pat. No. 3,331,970 and thus the system of FIG. 1 provides an intenseaudio signal when switch 12 is closed.

Referring now to FIG. 2 a preferred embodiment of the inventionfollowing the concepts of FIG. 1 will be described. In' the arrangementof FIG. 2 the power supply is connected via lead 21 to resistors 22 and23. Resistor 23 is connected in series circuit with resistor 24 acrossthe power supply so that the mid-point 25 thereof is at a voltagedependent on the relative values of resistors 23 and 24. A programmableunijunction transistor (PUT) 26 has its control electrode 27 connectedto the point 25. The anode 28 is connected by lead 29 and resistor 22 tothe positive terminal of power supply 11.

Capacitor 30 is connected between resistors 22 and 31 and forms part ofthe timing circuit referred to in FIG. 1. The piezoelectric transducer10 which also forms part of the diaphragm of an audio signal generatoris connected between points 25 and 32. It will be seen that when switch12 is closed current is provided to the piezoelectric transducer viaresistors 23 and 31. Thus it is stressed and moves in the manner wellknown in the art. When the switch 12 is closed the capacitor 30 alsostarts to charge at a rate controlled by resistor 22. Thus the voltageon the anode of the PUT 26 rises in a substantially linear fashion. Whenthe voltage on the anode 28 exceeds that on the gate 27 the PUT conductscausing capacitor 30 to discharge and also discharging the voltageacross the transducer 10. The transducer is thus excited and starts apositive excursion of point 25. Two complete cycles are shown in FIG.2A, with the time T corresponding to point 25 starting to go positive.The voltage at gate electrode 27 is shown by waveform 27A and thevoltage on anode 28 shown by waveform 28A. The voltages are shownrelative to the negative voltage of supply 11. As is well known in theart the PUT will remain nonconductive when the gate is positive relativeto the anode and will start to conduct when the gate is negative withrespect to the anode.

Since the gate 27 is connected to the transducer the gate voltagefollows the generally sinusoidal waveform indicated with the PUTnon-conductive till time T Between times T and T the excited transducerundergoes a positive excursion and then starts taking point 25 negative.At time T the piezoelectric transducer is nearing the completion of acycle of oscillation, but due to friction and losses due to driving thediaphragm the excursion amplitude is diminishing. When the voltage atpoint 25 becomes less than the voltage on the anode 28 (at time T thePUT 26 conducts. Capacitor 30 therefore discharges to the negativevoltage of battery 11, and point 25 is also lowered to that voltage viathe PUT gate circuit. Thus resistor 24 is shunted and the negativeexcursion of the transducer receives an in-phase signal whichaccelerates the excursion. This small in-phase signal brings theexcursion to full amplitude so that oscillation is sustained. After thetransducer achieves its maximum negative excursion and starts in theopposite direction, the voltage at point 25 again rises above thevoltage of the anode 25 attime T and hence the PUT 26 is heldnonconductive during the major portion of the next cycle of thetransducer. The operation is then repeated with the voltage at point 25again falling below the voltage of the anode 28 at time T It will beseen that the oscillation of the transducer is thereafter sustained bythe repeated application of the boosting signals.

Since the switch element 26 conducts for only a small portion of eachcycle of the transducer, and the remaining portion of the time theswitch appears as a very high impedance, the circuit illustrated hasbeen found to draw very little current and permits the use of a largenumber of such systems on a single telephone bell ringing circuit.

During conduction of the PUT anode 28 and gate 25 are both switchednegative at the same time. The capacitor 30 thus drives point 32negative. However, re-

, sistor 31 performs somewhat of a cushioning effect for the transducerfrom the fast wave-shape of the switch, and thus in practice is has beenfound that the voltage across the transducer is substantiallysimusoidal.

FIG. 3 is a schematic circuit diagram of an arrangement similar to thatof FIG. 2' and also making use of a programmable unijunction transistoras the controlled switch. In the circuit of FIG. 3 resistors 43, 44 and45 are connected in series circuit across the power supply 11 with thepiezoelectric transducer 114]) being coniiected in parallel withresistor 45. Resistor 42 and capacitor 50 are connected in seriescircuit from the positive terminal of power supply II to one side of thetransducer 10. The other side of the transducer It is connected to thegate 47 of the PUT 46. The anode 48 is connected to the junction ofresistor 42 with capacitor 50.

The operation of the circuit shown in FIG. 3 is substantially the sameas the operation of the circuit of FIG. 2 with resistor 45 acting aspart of the discharge circuit for the crystal 10. As in the case of thecircuit of FIG. 2, when the piezoelectric element It) starts itspositive excursion, the voltage on the piezoelectric element holds thegate 47 positive with respect to the voltage on the anode 48 and hencethe PUT is held against conduction until the crystal is on the negativeportion of its excursion. Then in the manner previously described, thePUT becomes conductive so that the boosting signal is applied to thecrystal in the manner described above.

FIG. 4 shows a circuit substantially the same as the circuit of FIG. 3with the exception that the controlled switch in the circuit of FIG. 4is a silicon controlled switch (SCS). With the circuit arrangements ofFIGS.

3 and 4 the impedance of the power source is typically lower in the caseof FIG. 2, which is adapted for use with a high impedance power supply.

In the circuit of FIG. 5 resistors 64 and 65 are con nected across thepower supply and provide the bias voltage for the base of transistor 63.The emitter of the NPN transistor 63 is connected to the gate of the PUT66 and to one side of the piezoelectric transducer 10. The capacitor isconnected in series circuit with resistors 61 and 62 across the powersupply and is also connected to the other side of the piezoelectrictransducer 10. The anode of the PUT 66 is connected to the capacitor 70and to resistor 62. A second capacitor 67 connects the emitter of thetransistor 63 with its base. It will be seen that the circuit of FIG. 5corresponds in principle to the circuit of FIG. 2 with the transistor 63essentially taking the place of resistor 23 in FIG. 2. The transistor 63looks like a few'ohms on the positive cycles of the transducer 10, sinceduring that time the transistor conducts. However, during the negativecycles the transistor looks like several megohms. As in the case of FIG.2 will be seen that the PUT compares the signal from the timing circuit(which includes capacitor 70) with the slgnalfrom the transducer andcontrols the application of the small boosting signal in the smallmanner as previously described.

In the circuit arrangement of FIG. 6 a PNP transistor serves as thelevel detector and the NPN transistor 76 serves as the controlledswitch. As an aid to understanding the invention the various circuitcomponents are contained within the functional blocks formed by thedashed lines. It will be seen that the base of transistor 75 and thecollector of transistor 76 are connected to the junction of resistors 73and 74. The emitter of transistor 75 is connected to the positive powersupply terminal by the resistor 72. Resistor 71 connects one side of thetransducer to the negative power supply terminal and resistor 74connects the other side of the transducer as well as the base oftransistor 75 to the negative power supply. Capacitor connects one sideof the transducer to the emitter of the level detecting transistor 75.The operation of the circuit of FIG. 6 corresponds in general to theoperation of the circuit of FIG. 2. That is, when power is applied tothe circuit,

current momentarily flows through resistors 71 and 73, exciting thetransducer 10. At the same time the capacitor 80 starts to charge viaresistor 72. When the voltage on the emitter of transistor 75 exceedsthe voltage on the base, transistor 75 starts to conduct current throughresistor 72 and also from capacitor 80. Current is thus supplied to thebase of transistor 76 so that transistor 76 conducts drawing currentfrom the base circuit of transistor 75 and hence insuring completeconduction of both transistors. Resistor 74 is thus shunted bytransistor 76, and the piezoelectric transducer is therefore discharged,causing its complete excitation. it will be seen that capacitor 80 willalso be completely discharged through the emitter-collector junction oftransistor 75 and the base-emitter junction of transistor 76. Resistor72 does not provide sufficient current to hold transistors 75 and 76conductive, and therefore the circuit reverts to its originalnonconducting mode. When this occurs, the now excited transducer beginsa positive excursion on the side connected to the base of transistor 75,assuring non-conduction of transistor 75.'

At the same time, the capacitor 80 starts charging toward the positivevoltage of the power supply through resistor 72. The rate at whichcapacitor 80 is permitted to charge is adjusted so that the voltage onthe base of transistor 75 runs ahead of the voltage on the emitter oftransistor 75 (in the manner indicated in the wave diagrams of FIG. 2A)and hence the transistor 75 remains non-conductive until the negativeexcursion of the transducer occurs. During the negative excursion of thetransducer, the base of transistor 75 goes slightly negative withrespect to the rising voltage on the emitter, and hence the transistor75 acts as a level detecting circuit and voltage comparator, causingtransistor 76 to become conductive at the appropriate point in thenegative excursion of the transducer. Thus the small boosting signal isapplied to the transducer in the manner previously described.

While the values of the circuit components will vary in accordance witha particular circuit, the following values were used in the circuit ofFIG. 2: resistors 22 and 24 were each 1 megohm, resistor 23 was 100,000ohms, resistor 31 was 1,000 ohms, and capacitor was 0.002 microfarads.The power supply was 26 volts, although it has been found that thecircuits described herein will operate over a wide variation of voltageranges. For example, the circuit of FIG. 5 has been found to work withvoltages ranging from 1.5 volts to 160 volts.

lt will be recognized, of course, that while the circuits disclosed indetail are shown for applying a small booting" signal to the transduceron one side of each cycle, the concept can be utilized to apply aboosting signal to each side of the cycle. However, in practice 1 havefound that with the circuit arrangements illustrated, a very smallcurrent drain on the power supply results, even through a substantialvolume of audio energy is being generated.

While the invention has been described by reference to presentlypreferred embodiments, it is intended that the following claims willencompass those changes and modifications which become obvious to aperson skilled in the art as a result of teachings hereof.

What is claimed is:

1. An audio signal generator for operation from a power supply,comprising in combination: a transducer operative to provide a dampedoutput signal alternating between successively decreasing maximum valuesthereof when electrically energized; a switching means connected to saidtransducer for applying an in-phase, boosting signal to said transducer;means actuating said switching means when said output signal approachesa maximum value, said actuating means including means providing athreshold signal whose voltage is less than that of said maximum valueof said transducer output signal, and wherein said switching meansincludes a comparison means operable to provide said in-phase boostingsignal when the voltage on said transducer substantially equals thevoltage of said threshold signal; and means connecting the power supplyto said transducer, said switching means and said actuating means.

2. An audio signal generator as recited in claim 1, wherein saidtransducer comprises a piezoelectric element.

3. An audio signal generator as recited in claim 1, wherein saidthreshold signal means includes a timing circuit which produces anoutput voltage which increases with respect to time in synchronism withsaid output signal, and said comparison means in said switching meansincludes at least one semi-conductor element having first and secondelectrodes respectively connected to said timing circuit and to saidtransducer, said semi-conductor element remaining nonconductive untilthe voltages on said two electrodes become substantially equal.

4. An audio signal generator as recited in claim 3, wherein saidsemiconductor element comprises a programmable unijunction transistorhaving a gate electrode connected to said transducer element and ananode electrode connected to said timing means.

5. An audio signal generator as recited in claim 3, wherein saidsemiconductor element comprises a silicon controlled switch having agate electrode connected to said transducer element and an anodeelectrode connected to said timing means.

6. A signal generator comprising in combination: an electricallysensitive transducer means including a piezoelectric element; timingcircuit means; power supply means connected to said transducer means andtiming circuit means; and signal comparison and switch means connectedto said transducer means, said timing circuit means and said powersupply means, said timing circuit means including means providing asignal having a voltage which rises to the voltage on said transducermeans as the voltage on said transducer means is approaching a maximumvalue, and said comparison and switch means includes means operativewhen the voltage on said transducer means substantially equals thevoltage of said timing signal to apply an in-phase, boosting signal tosaid transducer means.

7. A signal generator as recited in claim 6, wherein said signalcomparison and switch means includes at least one semiconductor elementhaving first and second electrodes respectively connected to the timingcircuit means and to the transducer means, said element remainingnon-conductive until the voltages on said two electrodes becomesubstantially equal.

8. A signal generator as recited in claim 6, wherein said semiconductorelement is maintained nonconductive during at least 50 percent of eachcycle of said transducer means.

9. A signal generator as recited in claim 6, wherein said transducermeans forms part of a sonic signal generator.

M). A sonic signal generator comprising in combination: power supplymeans having first and second terminals; a transducer element includinga piezoelectric crystal having first and second electrodes; first andsecond impedance elements connecting said first and second electrodes tosaid first and second terminals of said power supply means; firstcircuit means including a capacitor having a first side connected tosaid first electrode, said first circuit means further including meansconnecting a second side of said capacitor to said first terminal ofsaid power supply means; and second circuit means including asemiconductor element having a first electrode connected to said secondterminal of said power supply means and second and third electrodesrespectively connected to said second side of said capacitor and to saidsecond electrode of said transducer element, said semiconductor elementthereby being maintained non-conductive until the voltages on its saidsecond and third electrodes are substantially equal.

11. A sonic signal generator as recited in claim it), wherein saidconnecting means in said first circuit means further comprises a thirdimpedance element connecting said capacitor to said power supply meansso that the rate of charge of said capacitor is less than the rate ofvoltage increase on said transducer element.

12. A sonic signal generator as recited in claim 10, wherein saidsemiconductor elementcomprises a programmable unijunction transistorhaving an anode electrode connected to said semiconductors secondelectrode and a gate electrode connected as said semiconductors thirdelectrode.

13. A sonic signal generator as recited in claim l0, wherein saidsemiconductor element comprises a sili con control switch having ananode electrode connected as said semiconductors second electrode and agate electrode connected as said semiconductors third electrode.

1. An audio signal generator for operation from a power supply,comprising in combination: a transducer operative to provide a dampedoutput signal alternating between successively decreasing maximum valuesthereof when electrically energized; a switching means connected to saidtransducer for applying an in-phase, boosting signal to said transducer;means actuating said switching means when said output signal approachesa maximum value, said actuating means including means providing athreshold signal whose voltage is less than that of said maximum valueof said transducer output signal, and wherein said switching meansincludes a comparison means operable to provide said in-phase boostingsignal when the voltage on said transducer substantially equals thevoltage of said threshold signal; and means connecting the power supplyto said transducer, said switching means and said actuating means.
 2. Anaudio signal generator as recited in claim 1, wherein said transducercomprises a piezoelectric element.
 3. An audio signal generator asrecited in claim 1, wherein said threshold signal means includes atiming circuit which produces an outPut voltage which increases withrespect to time in synchronism with said output signal, and saidcomparison means in said switching means includes at least onesemi-conductor element having first and second electrodes respectivelyconnected to said timing circuit and to said transducer, saidsemi-conductor element remaining non-conductive until the voltages onsaid two electrodes become substantially equal.
 4. An audio signalgenerator as recited in claim 3, wherein said semiconductor elementcomprises a programmable unijunction transistor having a gate electrodeconnected to said transducer element and an anode electrode connected tosaid timing means.
 5. An audio signal generator as recited in claim 3,wherein said semiconductor element comprises a silicon controlled switchhaving a gate electrode connected to said transducer element and ananode electrode connected to said timing means.
 6. A signal generatorcomprising in combination: an electrically sensitive transducer meansincluding a piezoelectric element; timing circuit means; power supplymeans connected to said transducer means and timing circuit means; andsignal comparison and switch means connected to said transducer means,said timing circuit means and said power supply means, said timingcircuit means including means providing a signal having a voltage whichrises to the voltage on said transducer means as the voltage on saidtransducer means is approaching a maximum value, and said comparison andswitch means includes means operative when the voltage on saidtransducer means substantially equals the voltage of said timing signalto apply an in-phase, boosting signal to said transducer means.
 7. Asignal generator as recited in claim 6, wherein said signal comparisonand switch means includes at least one semiconductor element havingfirst and second electrodes respectively connected to the timing circuitmeans and to the transducer means, said element remaining non-conductiveuntil the voltages on said two electrodes become substantially equal. 8.A signal generator as recited in claim 6, wherein said semiconductorelement is maintained non-conductive during at least 50 percent of eachcycle of said transducer means.
 9. A signal generator as recited inclaim 6, wherein said transducer means forms part of a sonic signalgenerator.
 10. A sonic signal generator comprising in combination: powersupply means having first and second terminals; a transducer elementincluding a piezoelectric crystal having first and second electrodes;first and second impedance elements connecting said first and secondelectrodes to said first and second terminals of said power supplymeans; first circuit means including a capacitor having a first sideconnected to said first electrode, said first circuit means furtherincluding means connecting a second side of said capacitor to said firstterminal of said power supply means; and second circuit means includinga semiconductor element having a first electrode connected to saidsecond terminal of said power supply means and second and thirdelectrodes respectively connected to said second side of said capacitorand to said second electrode of said transducer element, saidsemiconductor element thereby being maintained non-conductive until thevoltages on its said second and third electrodes are substantiallyequal.
 11. A sonic signal generator as recited in claim 10, wherein saidconnecting means in said first circuit means further comprises a thirdimpedance element connecting said capacitor to said power supply meansso that the rate of charge of said capacitor is less than the rate ofvoltage increase on said transducer element.
 12. A sonic signalgenerator as recited in claim 10, wherein said semiconductor elementcomprises a programmable unijunction transistor having an anodeelectrode connected to said semiconductor''s second electrode and a gateelectrode connected as said semiconductor''s third electrode.
 13. Asonic signal generator as recited in claim 10, wherein saidsemiconductor element comprises a silicon control switch having an anodeelectrode connected as said semiconductor''s second electrode and a gateelectrode connected as said semiconductor''s third electrode.